U.S. Pat. No. 4,304,182 shows a plate tensioning means in which the folds or edges of the printing plate are inserted into a slot extending in the axial direction of the form cylinder. The edges are bent down and are elastically pretensioned between the lateral surfaces of the slot or between one lateral surface and the second printing plate edge contacting the other lateral surface. The tensioning force serves to secure the plate edges against slipping out by frictional engagement. However, aluminum printing plates or copper printing plates do not possess a sufficient modulus of elasticity for applying the spring forces required for reliably securing the plate.
U.S. Pat. No. 1,949,132 likewise shows a plate tensioning means in which the plate edges are inserted into a slot extending in the longitudinal direction of the form cylinder. In this instance, a ridge or strip is located in the tensioning slot, the strip being supported by flexible tongues against the side wall of the tensioning slot in order to secure the first edge of the printing plate pressed into the tensioning slot during the ongoing tensioning process. This tensioning device is not suitable for automatic plate insertion.
One object of the present invention is to provide a plate tensioning device which tensions the printing plate in a reliable manner using simple means and which is suitable for automatic plate changing.
This object is met in a device for fastening a flexible printing plate on the form cylinder of a printing machine, this form cylinder including at least one slot having a base and parallel lateral surfaces and extending in the axial direction, comprising a plurality of leaf-shaped springs disposed in the slot so as to be distributed along its length, each spring having two legs which are pretensioned when inserted into the slot base and contact the lateral surfaces of the slot in a U-shaped manner, one leg being elongated, containing a depression and having a rounded end portion; and the edges of the printing plate being received between the elongated leg and the adjacent lateral surfaces. The slot of the form cylinder and the leaf-shaped springs located therein are simple to produce in an inexpensive manner. The slot can be constructed so as to be very narrow due to the slender design of the springs, so that the loss of printing surface caused by the slot can be kept very low. The spring, which can be produced from steel, applies the required spring force for its secure fit in the cylinder slot in the installed state, there being no special requirements for surface roughness. Further, the elongated leg supplies the necessary, proportioned tensioning force for reliably tensioning the printing plate edges, enabling the following edges to be tensioned subsequently. Also, the printing form protects the spring and the slot from dirt and aggressive media in that it covers them. Finally, the device enables an automatic exchange of plates, e.g. by means of a robot.